Two-dimensional MX Dirac materials and quantum spin Hall insulators with tunable electronic and topological properties

We propose a novel class of two-dimensional(2D)Dirac materials in the MX family(M=Be,Mg,Zn and Cd,X=Cl,Br and I),which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy scales(0.8–1.8 eV)and ultra-high Fermi velocities comparable to graphene.Spin-orbit coupling opens sizable topological band gaps so that these compounds can be effectively classified as quantum spin Hall insulators.The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field.Electronic structures of several members of the family are shown to host a Van-Hove singularity(VHS)close to the energy of the Dirac node.The enhanced density-of-states associated with these VHSs could provide a mechanism for inducing topological superconductivity.The presence of sizable band gaps,ultra-high carrier mobilities,and small effective masses makes the MX family promising for electronics and spintronics applications.

Surface-regulated triangular borophene as Dirac-like materials from density functional calculation investigation

By applying the first principles calculations combined with density functional theory (DFT), this study explored the optical properties, electronic structure, and structure stability of triangular borophene decorated chemically, B3X (X=F, Cl) in a systematical manner. As revealed from the results of formation energy, phonon dispersion, and molecular dynamics simulation study, all the borophene decorated chemically were superior and able to be fabricated. In the present study, triangular borophene was reported to be converted into Dirac-like materials when functionalized by F and Cl exhibiting narrow direct band gaps as 0.19 eV and 0.17 eV, separately. Significant light absorption was assessed in the visible light and ultraviolet region. According the mentioned findings, these two-dimensional (2D) materials show large and wide promising applications for future nanoelectronics and optoelectronics.

Unusual electronic structure of Dirac material BaMnSb_(2) revealed by angle-resolved photoemission spectroscopy

High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb_(2). All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around Γ and a strong spot at Y which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb_(2) is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb_(2) and search for novel properties in this Dirac material.

Two-dimensional hexagonal Zn3Si2 monolayer:Dirac cone material and Dirac half-metallic manipulation

The fascinating Dirac cone in honeycomb graphene,which underlies many unique electronic properties,has inspired the vast endeavors on pursuing new two-dimensional(2D)Dirac materials.Based on the density functional theory method,a 2D material Zn3Si2 of honeycomb transition-metal silicide with intrinsic Dirac cones has been predicted.The Zn3Si2 monolayer is dynamically and thermodynamically stable under ambient conditions.Importantly,the Zn3Si2 monolayer is a room-temperature 2D Dirac material with a spin-orbit coupling energy gap of 1.2 meV,which has an intrinsic Dirac cone arising from the special hexagonal lattice structure.Hole doping leads to the spin polarization of the electron,which results in a Dirac half-metal feature with single-spin Dirac fermion.This novel stable 2D transition-metal-silicon-framework material holds promises for electronic device applications in spintronics.

On the mystery of the absence of a spin-orbit gap in scanning tunneling microscopy spectra of germanene

Germanene,the germanium analogue of graphene,shares many properties with its carbon counterpart.Both materials are two-dimensional materials that host Dirac fermions.There are,however,also a few important differences between these two materials:(1)graphene has a planar honeycomb lattice,whereas germanene’s honeycomb lattice is buckled and(2)the spin-orbit gap in germanene is predicted to be about three orders of magnitude larger than the spin-orbit gap in graphene(24 meV for germanene versus 20μeV for graphene).Surprisingly,scanning tunneling spectra recorded on germanene layers synthesized on different substrates do not show any sign of the presence of a spin-orbit gap.To date the exact origin of the absence of this spin-orbit gap in the scanning tunneling spectra of germanene has remained a mystery.In this work we show that the absence of the spin-orbit gap can be explained by germanene’s exceptionally low work function of only 3.8 eV.The difference in work function between germanene and the scanning tunneling microscopy tip(the work functions of most commonly used STM tips are in the range of 4.5 to 5.5 eV)gives rise to an electric field in the tunnel junction.This electric field results in a strong suppression of the size of the spin-orbit gap.